1. Field of the Invention
The present invention relates to a display device of an active matrix type provided with a plurality of pixel electrodes arranged in a matrix and each connected to a thin film transistor (hereinafter referred to as a “TFT”) as a switching element, and more particularly to a liquid crystal display (LCD) having an improved gate line driver.
2. Description of the Related Art
FIG. 1 shows an equivalent circuit diagram for one pixel in an LCD. A pixel TFT 5 connected to a gate line 2 and a data line 4 is connected to a pixel electrode 6. The pixel electrode 6 forms a capacitor CLC with an opposite electrode Vcom with liquid crystal 11 interposed therebetween. A storage capacitor CSC is provided in parallel to the liquid crystal capacitor CLC to maintain a voltage applied to the pixel electrode 6. While it is intended that the circuit have the above-described configuration, a parasitic capacitor CGS is generated between the gate and the source (the pixel electrode 6) of the TFT and increase in capacitance of the parasitic capacitor CGS results in problems, such as fluctuation in potential of the pixel electrode 6 due to the effects of a gate voltage applied to the gate line 2. In conventional devices, the effects of the parasitic capacitor CGS are reduced by providing a storage capacitor CSC with sufficient capacitance to account for that of the parasitic capacitor CGS.
In recent years, LCDs have come to be widely used for display devices in portable electronic devices, such as, for example, for viewfinders in digital still cameras and digital video cameras. LCDs for such portable devices must be made fine with a reduced display size while maintaining the number of pixels.
When a display size is reduced, the area of the pixel electrode is reduced, as is the electrode for forming the storage capacitor CSC. As a result, the capacitances of the liquid crystal capacitor CLC and the storage capacitor CSC are decreased. On the other hand, because the processible minimum line width is fixed, it is difficult to reduce the capacitance of the parasitic capacitor CGS beyond a certain level. Thus, when the LCD is made fine, the parasitic capacitor CGS has a relatively greater capacitance as compared to the liquid crystal capacitor CLC and the storage capacitor CSC.
Such an increase in capacitance of the parasitic capacitor CGS gives rise to an increase of a so-called drop voltage ΔV, i.e. the potential of the pixel electrode fluctuates because it is pulled down by a fall of the gate voltage. When the drop voltage ΔV is increased, various problems occur, such as generation of a difference in luminance between columns when liquid crystal is driven by an alternating voltage, and deviation of a central value Vc of a voltage applied to the pixel electrode from the potential Vcom of the opposite electrode.